The present invention relates to a vacuum degassing method for molten glass flow capable of removing bubbles properly and effectively from a continuous flow of molten glass obtained by melting glass materials.
Heretofore, it has been common to utilize a refining procedure to remove bubbles generated in molten glass obtained by melting raw materials of glass in a melting furnace, prior to forming the molten glass by a forming apparatus, in order to improve the quality of formed glass products.
There has been known such a method that in the refining procedure, a refining agent such as sodium sulfate (Na2SO4) is previously added to raw materials of glass and the molten glass obtained by melting the raw materials containing a refining agent is stored and maintained at a predetermined temperature for a predetermined period, during which bubbles in the molten glass grow by the help of the refining agent, rise to the molten glass surface, and the bubbles are removed.
Further, there has been known a vacuum degassing method wherein molten glass is introduced into a vacuum atmosphere under a reduced pressure; under such reduced pressure condition, bubbles in a continuous flow of molten glass grow up and rise to the molten glass surface at which bubbles break and are removed, and the molten glass is taken out from the vacuum atmosphere.
In the above-mentioned vacuum degassing method, the molten glass flow is formed under a reduced pressure wherein bubbles contained in the molten glass grow in a relatively short time and rise to the surface by using buoyancy of the grown-up bubbles in the molten glass, followed by breaking the bubbles on the surface of the molten glass. In this way, the method can remove bubbles effectively from the molten glass surface. In order to remove bubbles effectively from the molten glass surface, it is necessary to provide a high rising velocity of bubbles so that the bubbles come to the molten glass surface under a reduced pressure condition. Otherwise, the bubbles are discharged along with the molten glass flow, with the result that a final glass product contains bubbles and is defective.
For this reason, it is considered that the pressure in the reduced pressure atmosphere for vacuum degassing should be small as possible to grow up bubbles and the rising velocity be increased whereby the effect of vacuum degassing is improved.
However, when the pressure in the reduced pressure atmosphere for vacuum degassing is lowered, numerous new bubbles sometimes generate in the molten glass and the bubbles rise to the molten glass surface to form a floating foam layer without breaking. A part of the foam layer may be discharged along with the molten glass flow to result a defect in the glass product. When a foam layer grows, the temperature of the upper surface of the molten glass decreases. The foam layer tends to hardly break whereby the foam layer will further develop. As a result, the inside of the vacuum degassing apparatus is filled with non-breaking bubbles. The foam layer fully filling the apparatus may be in contact with impurities on the ceiling of the apparatus; thus, it brings the impurities in the final glass product. Consequently, excessively lowering the pressure in the atmosphere for vacuum degassing is not preferred for an effective treatment for vacuum degassing.
Further, the rising velocity of the bubbles in molten glass is determined by the viscosity of the molten glass as well as the size of the bubble. Accordingly, it is considered that the lowering of the viscosity of the molten glass, or the raising of the temperature of the molten glass can raise bubbles to the surface effectively. However, when the temperature of the molten glass is excessively raised, there causes an active reaction with the material of flow path, such as refractory bricks, with which the molten glass contacts. It may lead to occurrence of new bubbles and dissolution of a part of material of the flow path into the molten glass, thus resulting in deterioration of the quality of glass products. Further, when the temperature of the molten glass is raised, the strength of the material of the flow path is decreased, whereby the service life of the flow path is shortened and an extra equipment such as a heater for maintaining the high temperature of the molten glass is required. As a result, in order to conduct a proper and effective vacuum degassing treatment, it is difficult to lower excessively the pressure for vacuum degassing and also to raise excessively a temperature of the molten glass.
In the vacuum degassing method where several restrictions are imposed, the following conditions for effective degassing has been reported (SCIENCE AND TECHNOLOGY OF NEW GLASSES, Oct. 16-17, 1991, pages 75-76).
In a vacuum degassing apparatus 40 for carrying out a vacuum degassing method for a molten glass flow as shown in FIG. 4, the number of bubbles (bubble density) in molten glass decreases to about 1/1,000, when molten glass at 1,320xc2x0 C. is passed in the apparatus at a flow rate of 6 [ton/day] wherein a pressure in a vacuum degassing vessel 42 is 0.18 atom (136.8 mmHg) and a staying time of the molten glass in the vacuum degassing vessel 42 under such a reduced pressure atmosphere is 50 minutes.
Namely, the above-mentioned vacuum degassing treatment is conducted in a bench scale type vacuum degassing apparatus 40 in the following way. The molten glass obtained by melting raw materials of glass is introduced from an upstream pit 47 into the vacuum degassing vessel 42 under a reduced pressure via an uprising pipe 44 by a vacuum pump (not shown), whereby a molten glass flow is formed in a substantially horizontal direction. Then, the molten glass is passed in the vacuum degassing vessel 42 under a reduced pressure to remove the bubbles in the molten glass, and then, the molten glass is fed via a downfalling pipe 46 to a downstream pit 48 where the temperature of the molten glass is maintained to have the viscosity of 1,000 poises.
The molten glass is sampled at the inlet of the uprising pipe 44 and the outlet of the downfalling pipe 46 to check the bubble density contained in each sample of the molten glass. As a result, the bubble density contained in the molten glass in the upstream pit 47 prior to a vacuum degassing is 150 [number/kg] and the bubble density contained in the molten glass in the downstream pit 48 is 0.1 [number/kg]. Thus, it is recognized that the number of the bubbles decreases to about 1/1,000. It is also reported that a foam layer is not formed on the molten glass surface, although the pressure in the vacuum degassing vessel 42 is set to be low level as 0.18 atom.
The above-mentioned report discloses the vacuum degassing method wherein an effective vacuum degassing is attained when a pressure in the vacuum degassing vessel 42 is 0.18 atom (136.8 mmHg) and a staying time in the vacuum degassing vessel 42 is 50 minutes. However, it does not disclose various condition requirements for the vacuum degassing in order to obtain effectively superior results of vacuum degassing.
In particular, a vacuum degassing treatment should be carried out within a relatively short time under a reduced pressure atmosphere. Accordingly, under such conditions where the pressure in the reduced atmosphere can not be lowered excessively and the temperature of the molten glass can not be excessively high, as mentioned above, it is important to determine a staying time of the molten glass flow under the reduced pressure atmosphere.
The longer the staying time of the molten glass flowing in the vacuum degassing vessel 42, the uprising pipe 44 and the downfalling pipe 46, the lower the bubble density of the molten glass after vacuum degassing treatment.
In order to elongate the staying time of the molten glass under a reduced pressure atmosphere, it is considered to extend the length of flow path of the molten glass in a flow direction. However, this causes practical problems such as a remarkable increase in cost of the equipment due to the reasons as follow. Since an insulator for insulating a high temperature of the molten glass and a housing as a casing to maintain a reduced pressure, which surrounds the insulator and materials for the flow path, are provided at an outer periphery of the flow path for passing the molten glass of high temperature, the insulator and the housing must be extended according to the extension of the flow path. Further, a heavy structural unit comprising the materials for the flow path, the insulator and the housing must be movable so that the height of the unit can be adjusted depending on a pressure in the vacuum degassing vessel 42. This creates a large-sized movable equipment, hence, cost of the equipment will increase.
It is considered that the staying time can be extended by lowering the flow velocity of the molten glass. However, in order to lower the flow velocity, it is necessary to increase the viscosity by decreasing the temperature of the molten glass. In this case, it is difficult to raise the bubbles in the molten glass having a high viscosity to the molten glass surface.
On the other hand, when the staying time of the molten glass under a reduced pressure atmosphere is shortened excessively, sufficient degassing of the bubbles in the molten glass can not be achieved. Namely, a sufficient time for growing the bubbles in the molten glass under a reduced pressure atmosphere to raise them to the molten glass surface to thereby remove the bubbles by breaking can not be obtained, with the result that the molten glass with the bubbles may be discharged before the bubbles reach the molten glass surface. Although it is possible to lower the viscosity of the molten glass, i.e., to elevate the temperature of the molten glass in order to increase the rising velocity of bubbles in the molten glass, the temperature of the molten glass can not be increased because of the problems of a reduction in strength of the materials used for the flow path for the molten glass and the occurrence of new bubbles caused by the reaction of these materials with the molten glass.
It is an object of the present invention to provide a vacuum degassing method for molten glass flow, which is capable of obtaining effectively and certainly molten glass without containing bubbles by specifying a range of staying time of the molten glass in a case of conducting a degassing treatment to a continuous flow of molten glass under a reduced pressure atmosphere.
Further, the present invention aims at determining a proper range of vacuum degassing conditions for the molten glass under a reduced pressure atmosphere in the above-mentioned vacuum degassing method so that molten glass without containing bubbles can further be effectively and certainly obtained.
The inventors of this application have made extensive studies on vacuum degassing methods for molten glass flow to achieve the above-mentioned objects, and have found that it is necessary to make bubbles grown in molten glass to raise them to the molten glass surfaces where the breaking of the bubbles takes place, whereby the bubbles in the molten glass can effectively and certainly be removed. Thus, the present invention has been accomplished by satisfying the below-mentioned conditions:
1. The molten glass is continuously passed.
2. A condition that new bubbles are not generated is provided.
3. The diameter of bubbles is increased in a prescribed time so as to have a sufficient buoyancy.
4. The rising velocity of bubbles is provided to the bubbles so as to be against the molten glass flow.
5. A sufficient amount of gases to be diffused into the bubbles is assured so that the bubbles reaching the molten glass surface can be broken.
In accordance with the present invention, there is provided a vacuum degassing method for molten glass which comprises feeding, under an atmosphere of pressure P [mmHg], molten glass into a vacuum chamber capable of rendering a pressure to the molten glass to be in a range of 38 [mmHg]-(P-50) [mmHg] to perform degassing to the molten glass, and discharging the molten glass after having been degassed from the vacuum chamber at a flow rate of Q [ton/hr] under the atmosphere of pressure P [mmHg] wherein a staying time of the molten glass in the vacuum chamber is in a range of 0.12-4.8 hours, which is obtained by dividing a weight W [ton] of the molten glass flowing in the vacuum chamber by a flow rate Q [ton/hr] of the molten glass. In this case, the staying time in the vacuum chamber is preferably not less than 0.12 hour but not more than 0.8 hour.
Further, the vacuum chamber preferably includes a vacuum degassing vessel in which the molten glass is passed in a substantially horizontal state and is degassed, and a depth H [m] of the molten glass in the vacuum degassing vessel and a weight W [ton] of the molten glass satisfy the below-mentioned Formula (1):
0.010 m/ton less than H/W less than 1.5 m/ton.xe2x80x83xe2x80x83(1)
Further a surface area S1 [m2] of the molten glass surface in the vacuum degassing vessel and a flow rate Q [ton/hr] of a molten glass flow preferably satisfy the below-mentioned formula (2):
0.24 m2xc2x7hr/ton less than S1/Q less than 12 m2xc2x7hr/tonxe2x80x83xe2x80x83(2)
Further, the vacuum chamber preferably includes a downfalling pipe connected to the vacuum degassing vessel to discharge the molten glass therethrough, and a surface area S2 [m2] of flow path of the downfalling pipe at the portion where the downfalling pipe is connected to the vacuum degassing vessel and a flow rate Q [ton/hr] of the molten glass satisfy the below-mentioned Formula (3):
xe2x80x830.008 m2xc2x7hr/ton less than S2/Q less than 0.96 m2xc2x7hr/tonxe2x80x83xe2x80x83(3)